This invention deals with slender injection-molded plastic articles of the type having highly flexible internal steel strap reinforcements, and has particular application to playground swing seats which ay bend elastically through a 180.degree. arc.
An injection-molded swing seat having separate steel strap reinforcements is shown in U.S. Pat. No. 3,897,056. In it, the undersurface of the seat is molded to provide an elongated flat framed area just inward of the front edge, and a similar framed area just inward of the rear edge. These framed areas are crossed at intervals by molded ties or "keepers" which generally resemble belt loops. After molding the plastic article, steel straps are slid lengthwise beneath the molded ties and seated in the framed areas. At each of its ends, the steel straps have attachment apertures, which register with attachment apertures in the molded seat.
In that construction the straps are left exposed, so that they might rust if no protective coating was provided. Of greater concern is the possibility that the molded ties which hold the strap may break as when the seat is twisted or subjected to other misuse.
Persons familiar with the problems of injection molding will recognize the difficulty of fully encapsulating a flexible strap in a slender flexible article such as a swing seat. It is a familiar practice to use a minimum number of locator pins to substantially clamp relatively rigid steel inserts in position in the molds. In such case the pins rigidly prevent displacing of the insert by the inflow of plastic material. The present situation is quite different. The problem is not merely to retain the insert, but to prevent it from bending, as in a sine wave, or buckling or being otherwise distorted; while nevertheless permitting the inflow beneath the pin tips of enough plastic to form a continuing film over the insert, preserving it from corrosion.
Instead of flat strap,, steel cables, plated to prevent corrosion, have been molded in slender swing seats.